50. On the UNEQUAL 
detection of this fingular property of the acid of fea-falt, be- 
caufe, in making the fame experiment before, this inverfion of 
the order of the colours had entirely efcaped me. I was then 
examining it, to find whether it difperfed the feveral orders of 
rays, in the fame ratio in which glafs does ; and being fatisfied 
that it did not, from obferving the green and purple fringes, as 
in other combinations, a circumftance fo little looked for, as 
the inverfion of the order of the colours, did not ftrike me. 
Tus obfervation affords a remarkable exception to what I 
had begun to confider as very probably a general law of -na- 
ture. In the refraétion which takes place in mediums of the 
leaft difperfive kind, the green rays, or rather perhaps the rays 
in the confine of green and blue, are the mean refrangible, and 
thefe fame rays, in the more difperfive mediums, were always 
found among the lefs refrangible rays ; and hence when, by a 
proper combination of two fuch mediums, the red and violet 
rays are united, thefe united red and violet rays conftitute the 
leaft refrangible rays, and the green conftitute the moft refran- 
gible rays, as before explained. 
Bur in the muriatic acid, the cafe is juft the reverfe of this. 
Then the green rays, which in mediums that difperfe the leaft, 
are the mean refrangible, and which in effential oils and metal- 
lic impregnations are found among the lefs refrangible, appear 
amongft the more refrangible. Whence in fuch a combination - 
of the muriatic acid and an indifperfive medium as fhall. unite 
the red and violet rays, thefe united red and violet rays emerge 
moft refrangible, and the homogeneal green rays emerge leaft 
refrangible, being juft the reverfe of what takes place in com- 
binations of crown-glafs with flint-glafs, or with eflential oils, 
or faturated metallic folutions. 
Tus unufual property of the marine acid does not, how- 
ever, feem to admit of any immediate application to the im- 
provement of optical inftruments. It is true that, inftead of 
having recourfe to a compound concave for correcting the fe- 
. condary 
